WHILE FLYING STRAIGHT AND LEVEL AT 3100 FEET MSL, OVER VIRGINIA ROUTE 66 AND ROUTE 245, THE ENGINE BEGAN TO VIBRATE TO THE EXTENT THAT THE PLT THOUGHT THAT IT MIGHT SEPARATE FROM THE AIRFRAME.THE PLT SELECTED A FIELD AND SET UP AN APPROACH. ON FINAL, THE PLT DECIDED THAT THE FIELD WAS UNSUITABLE FOR LANDING. PWR WAS APPLIED AND THE PLT EXECUTED A 180 DEGREE TURN TO AN ALTERNATE FIELD. ACCORDING TO THE PLT, HIS AIRSPEED WAS TOO HIGH TO STOP IN THE SECOND FIELD, AND HE TURNED RIGHT ABOUT 45 DEGREES, AIMING FOR A NARROW BREAK IN THE TREE LINE. AFTER SEVERING THREE SMALL TREES, THE ACFTNOSED OVER AND ROTATED TO A STOP, COMING TO REST IN AN UPRIGHT POSITION. ACCORDING TO THE PLT, AN ENGINE MOUNT JOINT WASCRACKED IN THE WELD ITSELF, AND MOST OF THE FRACTURE SURFACES APPEARED AS IF A CRACK EXISTED PRIOR TO THE ACCIDENT FLIGHT.